1. Field of the Invention
The present invention relates to a developing agent for use in copying machines, printers, etc. and, more particularly, a developing agent wherein the quantity of charge is adjusted by metallic oxide fine particles so that the developing agent can exhibit stable charging characteristics even when continuously used for a long time.
2. Description of the Prior Art
It has been a conventional technique that the quantity of charge in an electro-photographic developing agent is adjusted by a charge control agent, such as nigrosine or quaternary ammonium salt. With the technical progress of the art of after-treatment agent, however, it has become possible to adjust the quantity of charge by an after-treatment agent. Methods available for this purpose include a method in which two or more different kinds of after-treatment agents are added so that charge adjustment is made according to the ratio of such addition, and another method in which charge adjustment is made by adding an after-treatment agent having its surface treated with a coupling agent. For example, Japanese Patent Publication No. 53-22447 describes a positive charge-controllable developing agent containing aminosilane-treated metallic oxide particles as a component material thereof.
With a method using such after-treatment agent for surface modification, however, it is not possible to achieve surface stability and to maintain the initial charging characteristics of the agent for long due to changes and deterioration in properties of the treated surface in the course of long continuous use. Another problem is that even when two or more different kinds of after-treatment agents are used, it is difficult to maintain the initial charging characteristics because of the fact that one kind of after-treatment agent is selectively disengaged alone from the toner particle surface with the result that the electrostatic charge goes out of balance. When the charging stability is low in this way, fogging will occur on a reproduced image.